Wednesday, 4 July 2012

A decade of operations and a few close approachers

The end of May marked 10 years of operation at Great Shefford Observatory, first light occurring on 26 May 2002 with the first astrometry being  measured on 30 May 2002, of Comet C/2000 WM1 (LINEAR). In the intervening 10 years I've taken 831,500 images and measured 30,600 astrometric positions of minor planets and comets, with many of the images being taken for photometry rather than astrometry.

The weather in April and May this year was particularly poor and looking at the stats over the last 10 years, this April was the worst April in all that time, with only 11 nights usable, nearly half of what would normally be expected. Indeed the 18-night cloudy period from April 23 - May 10 was easily the longest endured here, the previous worst being a period of 12 consecutive cloudy nights in January 2011.

In what remained of the month there were a number of notable close approachers observed, first up was 2012 JU, discovered at 6am UT on May 12th by the Catalina Sky Survey, 31 hours before passing by at 0.53 Lunar Distances (LD). David Briggs at South Observatory, Clanfield (code J84) reported positions during the evening of the 12th at mag +17 and I first picked it up at 23:15 UT, following it for just over 2 hours. In the 3 hours that David and I had it under observation its distance from Earth decreased by 1/5th, to 1.6 LD and its apparent speed accelerated from 60"/min to 90"/min. It was last reported from the Astronomical Research Observatory, Westfield, USA (observatory code H21) at 6h UT on 13th at 1.04 LD, moving at 208"/min at mag +16.

A much better observed fly-by was 2012 KP24, discovered from Mt. Lemmon with the 1.5-m reflector on May 23rd, giving NEO observers the luxury of 5 days warning before closest approach, due at 15:21 UT on May 28th, just 4 Earth diameters from the Earth's surface! At discovery it was mag +21 and 15 LD from Earth. I observed it on four consecutive nights, initially moving at just 1"/min and at mag +20 on May 24/25 but brightening about a magnitude each night. My last measurement was mag +16 in strong twilight at 02:40 UT on May 28th, with it still only moving at 30"/min even though by then it was only 1.5 LD away. It was last reported at 10:24 UT that morning, by the Catalina Sky Survey at mag +14 and moving at 187"/min, 5 hours before closest approach. Sergio Foglia posted on the Minor planet Mailing list a lightcurve derived from observations from H21 that shows a rotation period of 150 seconds and an amplitude of about 1 magnitude for this object, estimated to have a diameter between 13-19 metres.

However, even before 2012 KP24 was out of the way, the Mt. Lemmon telescope had picked up the next incoming object, 2012 KT42. The discovery image was taken just 23h 58m before 2012 KT42 would skim the Earth's surface by a mere 1.1 Earth diameters. As with 2012 KP24 the timing of the close approach would favour observatories in the USA more than those in the UK, with closest approach at 07:07 UT on May 29.

At discovery 2012 KT42 was at 3.6 LD and within 1.3° of the opposition point, about midway between Antares and Beta Sco and headed almost exactly towards Earth. From Great Shefford it was low in the south that night and I had to wait for it to pass into gaps between trees but eventually managed to image it for a 13 minute period a few minutes after midnight early on May 29th and again from 01:17-02:05 UT. It was mag +15 but only at 13° altitude by 2am.
2012 KT42, mag. +15.5, moving 49"/min, Distance from Earth 0.8 LD
Stack of 77 x 4 second exposures, 01:31:30 - 01:45:56 UT 29 May 2012
0.40-m Schmidt-Cassegrain at f/6, 2.1"/pixel, field 9.4'x7.7', North up.
Even though it was nearly 19 hours after discovery it had only moved about 3° from the discovery position, still headed almost straight at us, but had approached to just 0.76 LD by then. Mt. Lemmon remarkably managed to keep it under observation to within an hour of closest approach, their last reported astrometry was at 06:17 UT when 2012 KT42 would have been about 12th mag and just 3.6 Earth diameters above the Earth's surface, screaming along at 1,630"/min, equivalent to the apparent diameter of the moon in less than 70 seconds!

The alignment of 2012 KT42 during this approach was quite interesting, as already mentioned, it was nearly at the opposition point during most of the approach, but was actually in the Earth's penumbra from about 14:10 to 21:40 UT on May 28th. After close approach the line-up was still perfect and 2012 KT42 was in transit in front of the Sun for about an hour either side of 10:12 UT on May 29th, though far too small at about 0.006" diameter to be visible from Earth.